Effect of substrate wettability and flexibility on the initial stage of water vapor condensation
Understanding the mechanisms of controlling vapor condensation on surfaces is of significant importance in many fields. Despite many efforts made in the investigation of vapor condensation, few studies concern the condensation on flexible substrate, especially in microscale. In this paper, the effect of substrate wettability and flexibility on the initial stage of vapor condensation at the nanoscale is investigated. For this purpose, the condensation of high temperature water vapor on rigid and flexible substrates with different wettabilities are performed using molecular dynamics simulation. The results indicate that when substrates with the same flexibility vary from the hydrophobic to the hydrophilic, the condensation rate increases and the condensation mode changes from no-condensation, dropwise condensation, incomplete filmwise condensation to filmwise condensation, meanwhile, the heat exchange between the water vapor and the substrate becomes more efficient; when substrates with the same wettability vary from the rigid to the flexible, the nucleation density, the condensation rate and the heat exchange efficiency increases significantly. In addition, the condensation modes on rigid and flexible substrates with the same wettability are generally the same except for the substrates with εwater-Cu = 0.4 kcal/mol. And the critical values of substrate spring constant for condensation mode transition are about 80~100 kcal·mol-1·Å-2 when εwater-Cu = 0.4 kcal/mol. Therefore, changing the flexibility of the substrate is proposed as a new way to control the condensation mode at the initial stage of water vapor condensation to meet design requirements.